On-line synthesis and regenerating of a catalyst used in autothermal oxidation

ABSTRACT

An on-line method of synthesizing or regenerating catalysts for autothermal oxidation processes, specifically, the oxidation of paraffinic hydrocarbons, for example, ethane, propane, and naphtha, to olefins, for example, ethylene and propylene. The catalyst comprises a Group 8B metal, for example, a platinum group metal and, optionally, a promoter, such as tin, antimony, or copper, on a support, preferably a monolith support. On-line synthesis or regeneration involves co-feeding a volatile Group 8B metal compound and/or a volatile promoter compound with the paraffinic hydrocarbon and oxygen into the oxidation reactor under ignition or autothermal conditions.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/099,041, filed Sep. 3, 1998, U.S. Provisional Application Ser.No. 60/111,861, filed Dec. 11, 1998, and U.S. Provisional ApplicationSer. No. 60/136,003, filed May 26, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to catalytic autothermal oxidationprocesses. More particularly, the present invention relates to a methodof regenerating a catalyst used in the catalytic partial oxidation ofparaffinic hydrocarbons, such as ethane, propane, and naphtha, toolefins, such as ethylene and propylene.

Olefins find widespread utility in industrial organic chemistry.Ethylene is needed for the preparation of important polymers, such aspolyethylene, vinyl plastics, and ethylene-propylene rubbers, andimportant basic chemicals, such as ethylene oxide, styrene,acetaldehyde, ethyl acetate, and dichloro-ethane. Propylene is neededfor the preparation of polypropylene plastics, ethylene-propylenerubbers, and important basic chemicals, such as propylene oxide, cumene,and acrolein. Isobutylene is needed for the preparation of methyltertiary butyl ether. Long chain mono-olefins find utility in themanufacture of linear alkylated benzene sulfonates, which are used inthe detergent industry.

Low molecular weight olefins, such as ethylene, propylene, and butylene,are produced almost exclusively by thermal cracking (pyrolysis/steamcracking) of alkanes at elevated temperatures. An ethylene plant, forexample, typically achieves an ethylene selectivity of about 85 percentcalculated on a carbon atom basis at an ethane conversion of about 60mole percent. Undesired coproducts are recycled to the shell side of thecracking furnace to be burned, so as to produce the heat necessary forthe process. Disadvantageously, thermal cracking processes for olefinproduction are highly endothermic. Accordingly, these processes requirethe construction and maintenance of large, capital intensive, andcomplex cracking furnaces. The heat required to operate these furnacesat a temperature of about 900° C. is frequently obtained from thecombustion of methane which disadvantageously produces undesirablequantities of carbon dioxide. As a further disadvantage, the crackersmust be shut down periodically to remove coke deposits on the inside ofthe cracking coils.

Catalytic processes are known wherein paraffinic hydrocarbons areoxidatively dehydrogenated to form mono-olefins. In these processes, aparaffinic hydrocarbon is contacted with oxygen in the presence of acatalyst consisting of a platinum group metal or mixture thereofdeposited on a ceramic monolith support, typically in the form of ahoneycomb or foam. Optionally, hydrogen may be a component of the feed.The catalyst, prepared using conventional techniques, is uniformlyloaded throughout the support. The process can be conducted underautothermal reaction conditions wherein the feed is partially combusted,and the heat produced during combustion drives the endothermic crackingprocesses. Consequently, under autothermal process conditions there isno external heat source required; however, the catalyst is required tosupport combustion above the normal fuel-rich limit of flammability.Representative references disclosing this type of process include thefollowing U.S. Pat. Nos.: 4,940,826; 5,105,052; 5,382,741; and5,625,111. Disadvantageously, substantial amounts of deep oxidationproducts, such as carbon monoxide and carbon dioxide, are produced, andthe selectivity to olefins remains too low when compared with thermalcracking. Moreover, the references are silent with respect to a methodof regenerating the catalyst.

M. Huff and L. D. Schmidt disclose in the Journal of Physical Chemistry,97, 1993, 11,815, the production of ethylene from ethane in the presenceof air or oxygen under autothermal conditions over alumina foammonoliths coated with platinum, rhodium, or palladium. A similar articleby M. Huff and L. D. Schmidt in the Journal of Catalysis, 149, 1994,127-141, discloses the autothermal production of olefins from propaneand butane by oxidative dehydrogenation and cracking in air or oxygenover platinum and rhodium coated alumina foam monoliths. Again, theolefin activity achieved in these processes could be improved. Thereferences are also silent with respect to a method of regenerating thecatalyst.

U.S. Pat. No. 5,639,929 teaches an autothermal process for the oxidativedehydrogenation of C₂ -C₆ alkanes with an oxygen-containing gas in afluidized catalyst bed of platinum, rhodium, nickel, or platinum-goldsupported on alpha alumina or zirconia. Ethane produces ethylene, whilehigher olefins produce ethylene, propylene, and isobutylene. Again, theolefin selectivity could be improved, and the reference is silent withrespect to a method of regenerating the catalyst.

C. Yokoyama, S. S. Bharadwaj and L. D. Schmidt disclose in CatalysisLetters, 38, 1996, 181-188, the oxidative dehydrogenation of ethane toethylene under autothermal reaction conditions in the presence of abimetallic catalyst comprising platinum and a second metal selected fromtin, copper, silver, magnesium, cerium, lanthanum, nickel, cobalt, andgold supported on a ceramic foam monolith. The use of a catalystcomprising platinum with tin and/or copper results in an improved olefinselectivity; however, over time at high operating temperatures thesecond metal vaporizes off the catalyst and catalytic activitydecreases. When this occurs the reactor must be shut down to replace orregenerate the catalyst.

In view of the above, it would be desirable to discover an autothermalcatalytic process of oxidizing a paraffinic hydrocarbon to an olefinwherein the catalyst can be readily regenerated. Such a process wouldprovide the benefits of catalytic autothermal processes, such as lowlevels of catalyst coking and simplified engineering, with the addedbenefit of easy catalyst regenerability. It would be even more desirableif a catalytic autothermal process providing easy catalystregenerability was to achieve a paraffinic hydrocarbon conversion and anolefin selectivity comparable to those achieved by commercial thermalcracking processes.

SUMMARY OF THE INVENTION

This invention is a process of synthesizing or regenerating a catalystused in an autothermal catalytic oxidation process. In a preferredembodiment, the oxidation process involves contacting a paraffinichydrocarbon or a mixture of paraffinic hydrocarbons with oxygen in anoxidation reactor in the presence of the catalyst under autothermalprocess conditions sufficient to form at least one olefin. Hereinafter,the feed comprising the paraffinic hydrocarbon and oxygen, andoptionally hydrogen, may be referred to simply as the "reactantfeedstream" or, more simply, the "feedstream." The catalyst used in thisoxidation process comprises at least one Group 8B metal and, optionally,at least one promoter supported on a catalyst support, preferably amonolith support.

The catalyst synthesis/regeneration process of this invention isconducted "on-line," which means that the support, either blank or inthe form of a deactivated or partially deactivated catalyst, is loadedin the reactor and maintained under ignition or autothermal processconditions. The "blank" support is a fresh support absent any Group 8Bmetal and promoters.

The process of this invention, which involves synthesizing orregenerating a catalyst which is used in the autothermal oxidation ofparaffinic hydrocarbons to olefins, comprises feeding a volatile Group8B metal compound and/or a volatile promoter compound into the oxidationreactor simultaneously with the reactant feedstream under ignitionconditions or autothermal process conditions. Mixtures of volatile Group8B metal compounds and/or volatile promoter compounds can also beemployed. In the reactor the volatile Group 8B metal compound and thevolatile promoter compound contact the front face of the support wherethey decompose at the high temperature of the ignition or autothermalconditions into the corresponding Group 8B metal and/or promotercomponents.

The aforementioned method of this invention beneficially allows for thesynthesis of an oxidation catalyst on-line or alternatively, allows forthe regeneration of a deactivated or partially deactivated oxidationcatalyst on-line. The method of this invention eliminates the necessityof preparing the catalyst prior to loading the reactor and eliminatesthe necessity of shutting down the reactor to regenerate or replace thedeactivated catalyst. Additionally, novel catalyst compositions can beprepared and screened on-line for catalytic activity. The regenerationcan be beneficially employed on-line to replace metal components of thecatalyst which are lost over time through vaporization. Dead sections ofthe catalyst can be reactivated on-line. As a further advantage, themethod of this invention is readily engineered by simply introducing thevolatile compounds into the reactant feedstream. There is no necessity,for example, to construct complicated spray devices or separate portswithin the reactor.

Another advantage relates to reactor design. In one preferredembodiment, the reactor for this process comprises a housing, such as atube, into which the catalyst, in the form of catalytic componentsdeposited onto a monolith support, is packed. One or more radiationshields are typically packed on either side of the catalyst to reduceradiation heat losses. The radiation shield typically consists of a baremonolith support absent any catalytic metals. The entire reactor isinsulated to maintain essentially adiabatic conditions. The reactantfeedstream flows through an entrance port into the reactor, passesthrough the front radiation shield, and then contacts the catalyst. Theeffluent stream passes through the downstream radiation shield and exitsthe reactor. Advantageously, in the synthesis/regeneration process ofthis invention there is no necessity to remove the front radiationshield from the reactor, because the volatile Group 8B metal compoundand the volatile promoter compound used in the synthesis/regenerationprocess pass through the front radiation shield with the reactantfeedstream on the path to the catalyst. Moreover, uniform deposition ofthe volatile Group 8B metal compound and volatile promoter compound ontothe front edge of the catalyst may be achieved with minimum, if any,deposition onto extraneous surfaces.

All of the aforementioned advantages simplify the handling andmaintenance of the catalyst, reduce costs, and improve processefficiency.

In another aspect, this invention is an improved process of oxidizing aparaffinic hydrocarbon or mixture of paraffinic hydrocarbons to anolefin or a mixture of olefins. The process involves contacting aparaffinic hydrocarbon or mixture of paraffinic hydrocarbons and oxygenin an oxidation reactor in the presence of a catalyst under autothermalprocess conditions, and either continuously or intermittently feeding avolatile Group 8B metal compound and/or a volatile promoter compound, ora mixture thereof, into the reactor with the feedstream. The catalyst,as noted hereinbefore, comprises at least one Group 8B metal and,optionally, at least one promoter supported on a catalyst support,preferably a monolith support.

The autothermal oxidation process of this invention efficiently producesolefins, particularly mono-olefins, from paraffinic hydrocarbons andoxygen. Advantageously, the process of this invention achieves aparaffin conversion and olefin selectivity which are comparable tocommercial thermal cracking processes. As a further advantage, theprocess produces little, if any, coke, thereby substantially prolongingcatalyst lifetime and eliminating the necessity to shut down the reactorto remove coke deposits. The process of this invention employs a simpleengineering design thereby eliminating the requirement for a large,expensive, and complex furnace, such as those used in thermal crackingprocesses. More specifically, since the residence time of the reactantsin the process of this invention is on the order of milliseconds, thereaction zone used in this process operates at high volumetricthroughput. Accordingly, the reaction zone measures from aboutone-fiftieth to about one-hundredth the size of a commercially availablesteam cracker of comparable capacity. The reduced size of the reactorlowers costs and greatly simplifies catalyst loading and maintenanceprocedures. As a further advantage, since the process of this inventionis exothermic, the heat produced can be harvested via integrated heatexchangers to produce electrical energy or steam credits for otherprocesses. Finally, the improved autothermal oxidation process of thisinvention characterized by on-line regeneration of the catalyst achieveslong run times without interruption.

DETAILED DESCRIPTION OF THE INVENTION

The processes of this invention, described hereinafter, relate to theautothermal partial oxidation of paraffinic hydrocarbons to olefins. Thewords "partial oxidation" imply that the paraffinic hydrocarbon is notsubstantially oxidized to deep oxidation products, specifically, carbonmonoxide and carbon dioxide. Rather, the partial oxidation comprises oneor both of oxidative dehydrogenation and cracking to form primarilyolefins. It is not known or suggested to what extent or degree eitherprocess, oxidative dehydrogenation or cracking, predominates or occursto the exclusion of the other.

The oxidation process comprises contacting a paraffinic hydrocarbon ormixture of paraffinic hydrocarbons with oxygen in the presence of acatalyst under autothermal process conditions sufficient to form one ormore olefins. Optionally, the process can be conducted in the presenceof hydrogen, preferably, co-fed with the paraffinic hydrocarbon andoxygen. Together the paraffinic hydrocarbon and oxygen, and optionallyhydrogen, comprise the reactant feedstream. The catalyst employed in theprocess comprises at least one Group 8B metal and, optionally, at leastone promoter supported on a catalyst support, preferably a monolithsupport.

In one aspect, the process of this invention comprises a method ofsynthesizing or regenerating the aforementioned oxidation catalyst. Themethod comprises co-feeding a volatile Group 8B metal compound and/or avolatile promoter compound into the oxidation reactor simultaneouslywith the reactant feedstream under ignition or autothermal processconditions.

More specifically, the process of this invention comprises a method ofsynthesizing the aforementioned oxidation catalyst. The method comprisesco-feeding a volatile Group 8B metal compound and, optionally, avolatile promoter compound into the oxidation reactor simultaneouslywith the reactant feedstream. In the reactor the volatile Group 8B metalcompound and, optionally, the volatile promoter compound, contact ablank catalyst support and at the high ignition temperature decomposeinto the corresponding Group 8B metal and promoter, thereby forming theoxidation catalyst.

In another aspect, the process of this invention comprises regeneratingthe aforementioned oxidation catalyst after it has become deactivated orpartially deactivated. The process comprises co-feeding a volatile Group8B metal compound and/or a volatile promoter compound into the oxidationreactor simultaneously with the reactant feedstream. If the catalyst ispartially deactivated, then autothermal process conditions can beemployed. If the catalyst is fully deactivated, then ignition conditionscan be employed. The volatile compounds contact the deactivated orpartially deactivated catalyst and at the high temperature employeddecompose into the corresponding Group 8B metal and/or promoter, therebyregenerating the catalyst.

In the synthesis/regeneration process described hereinabove, it is alsoacceptable to employ more than one volatile Group 8B metal compoundand/or more than one volatile promoter compound. Moreover, in theprocess described hereinabove, the words "volatile Group 8B metalcompound" and "volatile promoter compound" are meant to include volatilecompounds in which the Group 8B metal or promoter are bonded to otherelements in a molecular composition. Additionally, the language is meantto include a vapor stream of the Group 8B metal and/or promoter in theirelemental form. Normally, one skilled in the art might not regard avapor stream of an element as a volatile "compound;" however, for thepurposes of this invention the term "volatile compound" will include theelemental vapor.

In another aspect, this invention is an improved autothermal process ofoxidizing paraffinic hydrocarbons to olefins. In this aspect, theinvention comprises contacting a paraffinic hydrocarbon or mixturethereof with oxygen in the presence of the aforementioned oxidationcatalyst under autothermal process conditions, and simultaneously,feeding into the reactant feedstream a volatile Group 8B metal compoundand/or a volatile promoter compound, or a mixture thereof, underautothermal process conditions.

In a preferred embodiment of this invention, the paraffinic hydrocarbonis selected from ethane, propane, mixtures of ethane and propane,naphtha, gas oils, vacuum gas oils, natural gas condensates, andmixtures of the aforementioned hydrocarbons; and the preferred olefinsare ethylene, propylene, butene, isobutylene, and butadiene.

In another preferred embodiment, the Group 8B metal is a platinum groupmetal; more preferably, the platinum group metal is platinum.

Typically, the promoter is selected from the elements and ions of Groups1B (Cu, Ag, Au), 6B (Cr, Mo, W), 3A (for example, Al, Ga, In), 4A (forexample, Ge, Sn, Pb), and 5A (for example, As, Sb, Bi) of the PeriodicTable of the Elements, as referenced by S. R. Radel and M. H. Navidi, inChemistry, West Publishing Company, New York, 1990. In a broadercontext, however, the promoter can include support modifiers, which aredefined as any ion having a charge of +1 or greater selected from Groups1A (Li, Na, K, Rb, Cs), 2A (for example, Mg, Ca, Sr, Ba), 3B (Sc, Y,La), 4B (Ti, Zr, Hf), 5B (V, Nb, Ta), 6B (Cr, Mo, W), 1B (Cu, Ag, Au),3A (for example, Al, Ga, In), 4A (for example, Ge, Sn, Pb), 5A (forexample, As, Sb, Bi), and the lanthanide rare earths (for example, Ce,Er, Lu, Ho) and actinide elements (for example, Th) of the PeriodicTable, previously referenced to S. R. Radel and M. H. Navidi, ibid.(Groups 1A, 2A, 3B, 4B, 5B, 6B, 1B, 3A, 4A, and 5A of the Periodic Tableare equivalent to Groups 1, 2, 3, 4, 5, 6, 11, 13, 14, and 15.) Thepreferred promoter is selected from the elements and ions of Groups 1B,6B, 3A, 4A, 5A, and the lanthanide rare earths. Mixtures of theaforementioned promoters can also be employed.

Any paraffinic hydrocarbon or mixture of paraffinic hydrocarbons can beemployed in the process of this invention provided that an olefin,preferably, a mono-olefin, is produced. The term "paraffinichydrocarbon", as used herein, refers to a saturated hydrocarbon.Generally, the paraffinic hydrocarbon contains at least 2 carbon atoms.Preferably, the paraffinic hydrocarbon contains from 2 to about 25carbon atoms, preferably, from 2 to about 15 carbon atoms, and even morepreferably, from 2 to about carbon atoms. The paraffinic hydrocarbon canhave a linear, branched, or cyclic structure, and can be a liquid or gasat ambient temperature and pressure. The paraffinic hydrocarbon can besupplied as an essentially pure paraffinic compound or as aparaffin-containing mixture of hydrocarbons. Paraffinic hydrocarbonfeeds which are suitably employed in the process of this inventioninclude, but are not limited to, ethane, propane, butane, pentane,hexane, heptane, octane, isomers and higher homologues thereof, as wellas complex higher boiling mixtures of paraffin-containing hydrocarbons,such as naphtha, gas oil, vacuum gas oil, and natural gas condensates.Additional feed components may include methane, nitrogen, carbonmonoxide, carbon dioxide, and steam, if so desired. Minor amounts ofunsaturated hydrocarbons may also be present. Most preferably, theparaffinic hydrocarbon is selected from ethane, propane, mixtures ofethane and propane, naphtha, gas oil, vacuum gas oil, natural gascondensates, and mixtures of the aforementioned hydrocarbons.

In the process of this invention, the paraffinic hydrocarbon iscontacted with an oxygen-containing gas. Preferably, the gas ismolecular oxygen or molecular oxygen diluted with an unreactive gas,such as nitrogen, helium, or argon, or diluted with a substantiallyunreactive gas, such as carbon monoxide or steam. Any molar ratio ofparaffinic hydrocarbon to oxygen is suitable, provided the desiredolefin is produced in the process of this invention. Preferably, theprocess is conducted fuel-rich and above the upper flammability limit. Afuel-rich feed reduces the selectivities to deep oxidation products,such as carbon monoxide and carbon dioxide, and beneficially increasesthe selectivity to olefins. Above the upper flammability limit,homogeneous (gas phase) combustion of the feed is not self-sustaining;therefore, the feed is safer to handle. One skilled in the art wouldknow how to determine the upper flammability limit for differentfeedstream mixtures comprising the paraffinic hydrocarbon, oxygen, andoptionally, hydrogen and diluent.

Generally, the molar ratio of paraffinic hydrocarbon to oxygen variesdepending upon the specific paraffin feed and autothermal processconditions employed. Typically, the molar ratio of paraffinichydrocarbon to oxygen ranges from about 3 to about 77 times thestoichiometric ratio of hydrocarbon to oxygen for complete combustion tocarbon dioxide and water. Preferably, the molar ratio of paraffinichydrocarbon to oxygen ranges from about 3 to about 13, more preferably,from about 4 to about 11, and most preferably, from about 5 to about 9times the stoichiometric ratio of hydrocarbon to oxygen for completecombustion to carbon dioxide and water. These general limits are usuallyachieved by employing a molar ratio of paraffinic hydrocarbon to oxygengreater than about 0.1:1, preferably, greater than about 0.2:1, and byusing a molar ratio of paraffinic hydrocarbon to oxygen usually lessthan about 3.0:1, preferably, less than about 2.7:1. For preferredparaffins, the following ratios are more specific. For ethane, theethane to oxygen molar ratio is typically greater than about 1.5:1, andpreferably, greater than about 1.8:1. The ethane to oxygen molar ratiois typically less than about 3.0:1, preferably, less than about 2.7:1.For propane, the propane to oxygen molar ratio is typically greater thanabout 0.9:1, preferably, greater than about 1.1:1. The propane to oxygenmolar ratio is typically less than about 2.2:1, preferably, less thanabout 2.0:1. For naphtha, the naphtha to oxygen molar ratio is typicallygreater than about 0.3:1, preferably, greater than about 0.5:1. Thenaphtha to oxygen molar ratio is typically less than about 1.0:1,preferably, less than about 0.9:1. One skilled in the art can adjust theaforementioned hydrocarbon/oxygen molar ratio to higher or lower valuesas may be dictated by the specific feed and autothermal processconditions. For example, if the feedstream is preheated to a temperaturegreater than about 200° C., the ratio of the paraffinic hydrocarbon tooxygen can be shifted towards higher values, up to about 4.0:1.

Optionally, hydrogen may be co-fed with the paraffinic hydrocarbon andoxygen to the catalyst. The presence of hydrogen in the feedstreambeneficially improves the conversion of hydrocarbon and the selectivityto olefins, while reducing the formation of deep oxidation products,such as, carbon monoxide and carbon dioxide. The molar ratio of hydrogento oxygen can vary over any operable range, provided that the desiredolefin product is produced. Typically, the molar ratio of hydrogen tooxygen is greater than about 0.5:1, preferably, greater than about0.7:1, and more preferably, greater than about 1.5:1. Typically, themolar ratio of hydrogen to oxygen is less than about 3.2:1, preferably,less than about 3.0:1, and more preferably, less than about 2.7:1. Thehydrogen to oxygen molar ratio may also be adjusted to higher or lowervalues to fit the specific feed and autothermal process conditions. Forexample, if the feedstream is preheated to a temperature greater thanabout 200° C., the hydrogen to oxygen molar ratio may be shifted tohigher values, up to about 4.0:1.

Optionally, the feed may contain a diluent, which can be any gas orvaporizable liquid which essentially does not interfere with theoxidation process of the invention. The diluent functions as a carrierof the reactants and products and facilitates the transfer of heatgenerated by the process. The diluent also helps to minimize undesirablesecondary reactions and helps to expand the non-flammable regime formixtures of the paraffinic hydrocarbon and oxygen, and optionallyhydrogen. Suitable diluents include nitrogen, argon, helium, carbondioxide, carbon monoxide, methane, and steam. The concentration ofdiluent in the feed can vary over a wide range. If a diluent is used,the concentration of diluent is typically greater than about 0.1 molepercent of the total reactant feed including paraffinic hydrocarbon,oxygen, diluent, and optional hydrogen. Preferably, the amount ofdiluent is greater than about 1 mole percent of the total reactant feed.Typically, the amount of diluent is less than about 70 mole percent, andpreferably, less than about 40 mole percent, of the total reactant feed.

The catalyst which is employed in the process of this inventionbeneficially comprises a Group 8B metal, and optionally, at least onepromoter supported on a catalyst support, preferably, a monolithsupport. The Group 8B metals include iron, cobalt, nickel, and theplatinum group elements, namely, ruthenium, rhodium, palladium, osmium,iridium, and platinum. Mixtures of the aforementioned Group 8B metalsmay also be used. Preferably, the Group 8B metal is a platinum groupmetal. Preferably, the platinum group metal is platinum.

The catalyst optionally comprises at least one promoter, which issuitably defined as any element or elemental ion which is capable ofenhancing the performance of the catalyst, as measured, for example, byan increase in the paraffin conversion, an increase in the selectivityto olefin, a decrease in the selectivities to deep oxidation products,such as carbon monoxide and carbon dioxide, and/or an increase incatalyst stability and lifetime. Typically, the term "promoter" does notinclude the platinum group metals. Broadly, the promoter can be selectedfrom Groups 1A, 2A, 3B, 4B, 5B, 6B, 1B, 3A, 4A, 5A, and the lanthaniderare earths and actinide elements of the Periodic Table, as previouslyreferenced by S. R. Radel and M. H. Navidi, ibid. Preferably, thepromoter is selected from Groups 1B, 6B, 3A, 4A, 5A, and the lanthanideelements. Mixtures of the aforementioned promoters can also be employed.More preferably, the promoter is selected from copper, tin, antimony,silver, indium, and mixtures thereof. Most preferably, the promoter iscopper, tin, antimony, or a mixture thereof. If a promoter is employed,then any atomic ratio of Group 8B metal to promoter in the freshcatalyst is suitable, provided that the catalyst is operable in theprocess of this invention. The optimal atomic ratio will vary with thespecific Group 8B metal and promoter employed. Generally, the atomicratio of Group 8B metal to promoter is greater than about 0.10 (1:10),preferably, greater than about 0.13 (1:8), and more preferably, greaterthan about 0.17 (1:6). Generally, the atomic ratio of the Group 8B metalto promoter is less than about 2.0 (1:0.5), preferably, less than about0.33 (1:3), and more preferably, less than about 0.25 (1:4). Althoughthe promoter may be used in a gram-atom amount equivalent to or greaterthan the Group 8B metal, the promoter nevertheless functions to enhancethe catalytic effect of the catalyst. Compositions prepared withpromoter alone, in the absence of the Group 8B metal, are typically (butnot always) catalytically inactive in the process. In contrast, theGroup 8B metal is catalytically active in the absence of promoter metal,albeit with lesser activity.

In one form, the catalyst can be supplied as a metallic gauze. In thisform, the gauze acts as both catalyst and monolith support. Morespecifically, the gauze can comprise an essentially pure Group 8B metalor an alloy of Group 8B metals, preferably, platinum group metals, ontowhich optionally a promoter is deposited. Suitable gauzes of this typeinclude pure platinum gauze and platinum-rhodium alloy gauze, optionallycoated with the promoter. The method used to deposit or coat thepromoter onto the gauze can be any of the methods described hereinafter.Alternatively, a gauze comprising an alloy of a Group 8B metal and thepromoter can be employed. Suitable examples of this type include gauzesprepared from platinum-tin, platinum-copper, and platinum-tin-copperalloys. During regeneration, one or more of the Group 8B alloy metalsand/or the same or a different promoter can be deposited.

In another embodiment, the Group 8B metal and promoter are supported ona catalytic support. The loading of the Group 8B metal on the supportcan be any loading which provides for an operable catalyst in theprocess of this invention. In general, the loading of the Group 8B metalcan be as low as about 0.0001 weight percent, based on the total weightof the Group 8B metal and support. Preferably, the loading of the Group8B metal is greater than about 0.1 weight percent, and more preferably,greater than about 0.2 weight percent, based on the total weight of theGroup 8B metal and the support. Preferably, the loading of the Group 8Bmetal is less than about 80 weight percent, preferably, less than about60 weight percent, and more preferably, less than about 10 weightpercent, based on the total weight of the Group 8B metal and thesupport. Once the Group 8B metal loading is established, the desiredatomic ratio of Group 8B metal to promoter determines the loading of thepromoter.

The catalytic support comprises any material which provides a surface tocarry the Group 8B metal, and optionally, any promoter(s) and supportmodifiers, as described hereinafter. Preferably, the support isthermally and mechanically stable under autothermal process conditions.The support may exhibit essentially no activity with respect to theoxidation process and may consequently be regarded as inert.Alternatively, the support may exhibit some reactivity with respect tothe oxidation process; for example, different supports may increase ordecrease the conversion of the paraffinic hydrocarbon and theselectivity to olefinic products.

Preferably, the support is a ceramic, such as a refractory oxide,nitride, or carbide. Non-limiting examples of suitable ceramics includealumina, silica, silica-aluminas, aluminosilicates, for example,cordierite, as well as, magnesia, magnesium aluminate spinels, magnesiumsilicates, zirconia, titania, boria, zirconia toughened alumina (ZTA),lithium aluminum silicates, silicon carbide, silicon nitride, andoxide-bonded silicon carbide. Mixtures of the aforementioned refractoryoxides, nitrides, and carbides may also be employed, as well as,washcoats of the aforementioned materials on a support. Preferredceramics include magnesia, alumina, silica, and amorphous andcrystalline combinations of alumina and silica, including mullite. Alpha(α) and gamma (γ) alumina are preferred forms of alumina. Preferredcombinations of alumina and silica comprise from about 65 to about 100weight percent alumina and from essentially 0 to about 35 weight percentsilica. Other refractory oxides, such as boria, can be present insmaller amounts in the preferred alumina and silica mixtures. Preferredzirconias include zirconia fully stabilized with calcia (SSZ) andzirconia partially stabilized with magnesia (PSZ), available fromVesuvius Hi-Tech Ceramics, Inc. Magnesia is the most preferred support,because it produces fewer cracking products and less carbon monoxide.Moreover, the hydrocarbon conversion and olefin selectivity tend to behigher with magnesia.

The catalytic support may take a variety of shapes including that ofporous or non-porous spheres, granules, pellets, irregularly shapedsolid or porous particles, or any other shape which is suitable for avariety of catalytic reactors, including fixed bed, transport bed, andfluidized bed reactors. In a preferred form, the catalyst is a monolith,which means that it is a continuous structure. Examples of monolithsinclude honeycomb structures, foams, and fibers woven into fabrics ormade into non-woven mats or thin paper-like sheets. Foams aresponge-like structures. More preferably, the support is a foam or fiberceramic monolith. Catalysts prepared with foam or fiber supports tend tohave a higher activity as compared with catalysts prepared on solidspheres or irregularly shaped particles. Additionally, fibers tend topossess higher fracture resistance as compared with foams andhoneycombs. Preferred ceramic foams, available from Vesuvius Hi-TechCeramics, Inc., comprise alpha alumina, zirconia, and mullite with aporosity ranging from about 5 to about 100 pores per linear inch (ppi)(2 to 40 pores per linear cm (ppcm)). Foams having about 45 ppi (18ppcm) are more preferred. The term "porosity," as used herein, refers tochannel size or dimension. It is important to note that the foamsupports are not substantially microporous structures. Rather, the foamsare macroporous, meaning that they are low surface area supports withchannels ranging in diameter from about 0.1 mm to about 5 mm. The foamsare estimated to have a surface area less than about 10 m² /g, andpreferably, less than about 2 m² /g, but greater than about 0.001 m² /g.

More preferred ceramic fibers, such as those available as Nextel® brandceramic fibers, a trademark of 3M Corporation, typically have a diametergreater than about 1 micron (μm), preferably, greater than about 5microns (μm). The diameter is suitably less than about 20 μm,preferably, less than about 15 μm. The length of the fibers is generallygreater than about 0.5 inch (1.25 cm), preferably, greater than about 1inch (2.5 cm), and typically less than about 10 inches (25.0 cm),preferably, less than about 5 inches (12.5 cm). The surface area of thefibers is very low, being generally less than about 1 m² /g, preferably,less than about 0.3 m² /g, but greater than about 0.001 m² /g.Preferably, the fibers are not woven like cloth, but instead arerandomly intertwined as in a mat or matted rug. Most preferred areNextel® brand 312 fibers which consist of alumina (62 weight percent),silica (24 weight percent), and boria (14 weight percent). Non-limitingexamples of other suitable fibers include Nextel® brand 440 fibers whichconsist of gamma alumina (70 weight percent), silica (28 weightpercent), and boria (2 weight percent) and Nextel® brand 610 fiberswhich consist of alpha alumina (99 weight percent), silica (0.2-0.3weight percent) and iron oxide (0.4-0.7 weight percent).

The catalyst of this invention can be synthesized "off-line," that is,outside the reactor, and then regenerated "on-line," that is, in situ.For the sake of thoroughness, the "off-line" synthesis is described indetail hereinafter. In "off-line" synthesis, the deposition of the Group8B metal and promoter onto the support can be made by any techniqueknown to those skilled in the art, for example, impregnation,ion-exchange, deposition-precipitation, vapor deposition, sputtering,and ion implantation. In one preferred "off-line" synthesis the Group 8Bmetal is deposited onto the support by impregnation. Impregnation isdescribed by Charles N. Satterfield in Heterogeneous Catalysis inPractice, McGraw-Hill Book Company, New York, 1980, 82-84, incorporatedherein by reference. In this procedure, the support is wetted with asolution containing a soluble Group 8B metal compound, preferably, tothe point of incipient wetness. The temperature of the depositiontypically ranges from about ambient, taken as 23° C., to about 100° C.,preferably, from about 23° C. to about 50° C. The deposition isconducted usually at ambient pressure. Non-limiting examples of suitableGroup 8B metal compounds include Group 8B metal nitrates, halides,sulfates, alkoxides, carboxylates, and Group 8B metal organometalliccompounds, such as halo, amino, and carbonyl complexes. Preferably, theGroup 8B metal compound is a platinum group metal compound, morepreferably, a platinum group metal halide, most preferably, a platinumgroup chloride, such as chloroplatinic acid. The solvent can be anyliquid which solubilizes the Group 8B metal compound. Suitable solventsinclude water, aliphatic alcohols, aliphatic and aromatic hydrocarbons,and halo-substituted aliphatic and aromatic hydrocarbons. Theconcentration of the Group 8B metal compound in the solution generallyranges from about 0.001 molar (M) to about 10 M. After contacting thesupport with the solution containing the Group 8B metal compound, thesupport may be dried under air at a temperature ranging from about 23°C. to a temperature below the decomposition temperature of the Group 8Bmetal compound, typically, a temperature between about 23° C. and about100° C.

The deposition of the promoter can be accomplished in a manner analogousto the deposition of the Group 8B metal. Accordingly, if impregnation isused, then the support is wetted with a solution containing a solublepromoter compound at a temperature between about 23° C. and about 100°C., preferably, between about 23° C. and about 50° C., at about ambientpressure. Suitable examples of soluble promoter compounds includepromoter halides, nitrates, alkoxides, carboxylates, sulfates, andpromoter organometallic compounds, such as amino, halo, and carbonylcomplexes. Suitable solvents comprise water, aliphatic alcohols,aliphatic and aromatic hydrocarbons, and chloro-substituted aliphaticand aromatic hydrocarbons. Certain promoter compounds, such as compoundsof tin, may be more readily solubilized in the presence of acid, such ashydrochloric acid. The concentration of the promoter compound in thesolution generally ranges from about 0.01 M to about 10 M. Followingdeposition of the soluble promoter compound or mixture thereof, theimpregnated support may be dried under air at a temperature betweenabout 23° C. and a temperature below the temperature whereinvaporization or decomposition of the promoter compound occurs.Typically, the drying is conducted at a temperature between about 23° C.and about 100° C.

In one method of preparing the catalyst, the Group 8B metal is depositedonto the support first, and thereafter the promoter is deposited ontothe support. In an alternative method, the promoter is deposited first,followed by the deposition of the Group 8B metal. In a preferred methodof preparing the catalyst, the Group 8B metal and the promoter aredeposited simultaneously onto the support from the same depositionsolution.

Following one or more depositions of the Group 8B metal and optionalpromoter compounds onto the support, a calcination under oxygen isoptional. If performed, the calcination is conducted at a temperatureranging from about 100° C. to below the temperature at whichvolatilization of the metals becomes significant, typically, atemperature less than about 1,110° C. Preferably, the calcination isconducted at a temperature between about 100° C. and about 500° C.

As a final step in the "off-line" preparation of the catalyst, the fullyloaded support is reduced under a reducing agent, such as hydrogen,carbon monoxide, or ammonia, at a temperature between about 100° C. andabout 800° C., preferably between about 125° C. and about 600° C., so asto convert the Group 8B metal substantially into its elemental metallicform. The promoter may be fully or partially reduced, or not reduced atall, depending upon the specific promoter chosen and the reductionconditions. In addition, reduction at elevated temperatures may producealloys of the Group 8B metal and the promoter. Alloys may provideenhanced catalyst stability by retarding vaporization of the promoterduring the process of this invention.

In another preferred embodiment of the "off-line" synthesis, the supportis pretreated with a support modifier prior to loading the Group 8Bmetal and promoter(s). The support modifier can be any metal ion havinga charge of +1 or greater. Preferably, the support modifier is selectedfrom Groups 1A, 2A, 3B, 4B, 5B, 6B, 1B, 3A, 4A, 5A, (the aforementionedbeing equivalent to Groups 1, 2, 3, 4, 5, 6, 11, 13, 14, 15), and thelanthanide rare earths and actinide elements (specifically thorium) ofthe Periodic Table, as previously referenced by S. R. Radel and M. H.Navidi, ibid. More preferably, the support modifier is selected fromcalcium, zirconium, tin, lanthanum, potassium, lutetium, erbium, barium,holmium, cerium, antimony, and mixtures thereof. Most preferably, thesupport modifier is selected from lanthanum, tin, antimony, calcium, andmixtures thereof. Certain elements, such as tin, antimony, and silver,may function as both promoter and support modifier simultaneously. Asnoted hereinbefore, for the purposes of this invention, the supportmodifier is included within the broad definition of the promoter.

The procedure to modify the support comprises contacting the supportwith a solution containing a soluble compound of the support modifier.The contacting can involve ion-exchange or impregnation methods.Preferably, the modification procedure involves submerging the supportin the solution such that essentially all of the surface area of thesupport is contacted with an excess of the solution. Compounds suitablefor preparing the solution of support modifier include modifiernitrates, halides, particularly the chlorides, alkoxides, carboxylates,and organometallic complexes including amino, halo, alkyl, and carbonylcomplexes. Suitable solvents include water, aliphatic alcohols, aromatichydrocarbons, and halo-substituted aliphatic and aromatic hydrocarbons.Typically, the concentration of modifier compound in the solution rangesfrom about 0.001 M to about 10 M. Acidified solutions, for example, ofhydrochloric acid and diluted solutions thereof, may be beneficiallyemployed. The contact time generally ranges from about 1 minute to about1 day. The contacting temperature suitably ranges from about 23° C. toabout 100° C., and pressure is generally ambient. The modified supportis typically calcined, as noted hereinabove, or reduced under a reducingagent, such as hydrogen, at a temperature between about 100° C. andabout 900° C., preferably, between about 200° C. and about 800° C. Thechoice of calcination or reduction depends on the element used topretreat the support. If the element or its oxide is readilyvaporizable, the pretreated support is reduced. If the element or itsoxide is not readily vaporizable, then the pretreated support can becalcined. As a guideline, the words "readily vaporizable" may be takento mean that greater than about 1 weight percent of any metal componentin the catalyst is vaporized in a period of about 24 h under calcinationconditions at about 200° C. The term "readily vaporizable" may be givena narrower or broader definition, as desired.

Following the pretreatment modification, the Group 8B metal and promoterare loaded onto the support. Then, the support is reduced as describedhereinbefore. Alternatively, the metal-loaded support may be calcinedfirst, as described hereinbefore, and then reduced. Whether the modifiedsupport is calcined or not depends again upon the vaporization potentialof the modifier metal(s) and promoter(s) employed. Supports modifiedwith metals which tend to vaporize readily are typically not calcined.Supports modified with metals that form a volatile oxide are typicallynot calcined. Supports modified with metals or metal oxides which do notvaporize readily can be calcined.

In another preferred aspect, the Group 8B metal and optional promoter(s)are loaded onto the front edge of the support, as opposed to beinguniformly loading throughout the support. Front face (or up-front)loading leads to improved selectivity to olefins in the oxidationprocess of this invention. If the support is not yet loaded into thereactor, front face loading can be accomplished by conventionaltechniques, such as, impregnation of the front face of a blank supportwith solutions containing the Group 8B metal and promoter(s).

In the method of interest in this invention, the catalyst is synthesizedor regenerated "on-line." On-line synthesis is accomplished byco-feeding at least one volatile Group 8B metal compound and,optionally, at least one volatile promoter compound into the reactorwith the reactant feedstream under ignition conditions. In this method,a blank support, defined as a fresh support absent any Group 8B metaland promoter(s), is positioned in the reactor and heated to atemperature sufficient to effect ignition. On-line regeneration issimilarly accomplished, with the exception that the deactivated orpartially deactivated catalyst is positioned in the reactor and heatedto autothermal conditions. On-line synthesis and regeneration yieldfront-face loaded catalysts, which are preferred.

In the on-line synthesis or regeneration method of this invention, theGroup 8B metal and/or promoter(s) can be deposited from a vapor streamof the metallic element(s). Alternatively, any chemical compoundcontaining the Group 8B metal and/or the promoter can be employed,provided the compound has sufficient volatility. The term "sufficientvolatility" means that the Group 8B metal compound and/or promotercompound can be volatilized under the preheat conditions of the reactantfeedstream, typically, a temperature between about 40° C. and about 550°C. The volatile compound(s) can be introduced continuously orintermittently into the reactant feedstream, as desired.

Non-limiting examples of suitable volatile Group 8B metal compounds andvolatile promoter compounds include volatile Group 8B and promotercomplexes containing ligands selected from carbonyl, halides, alkyls,mono-olefins, diolefins, acetylene, allyl, cyclo(hydrocarbyl)dienes,such as cyclobutadiene and cyclooctatetraene, cyclo(hydrocarbyl)dienyls,such as cyclopentadienyl, cycloheptatrienyl, as well as aryl ligands,such as benzene, and complexes containing mixed varieties of theseligands, that is, mixed variations. Also suitable are the volatilealkoxides, oxides, and phosphines. Preferably, the volatile Group 8Bmetal compound is a Group 8B carbonyl, phosphine, or olefin complex, ormixed variation thereof. More preferably, the volatile Group 8B metalcompound is selected from (trihalophosphine)platinum group metalcomplexes. Most preferably, the volatile platinum group compound istetrakis(trifluorophosphine)platinum (0). Preferably, the volatilepromoter compound is selected from promoter alkyl, amine, carbonyl,halide, and aryl complexes, and mixed variations thereof. Suitableexamples of the volatile promoter compound include tetraethyltin,dichloroditolylstannane, diethyldibromodipyridinetin, diethyltindibromide, diethyltin dichloride, dimethyldiethyltin,dimethylethylpropyltin, dimethyltin dichloride, dimethyltin dibromide,phenylbenzyltin dichloride, tribenzylethyltin, tribenzyltin chloride,tributyltin acetate, triethyltin chloride, triethyltin hydroxide,triphenylallyltin, triphenylbenzyltin, triphenylmethyltin,triphenylethyltin, triphenylbutyltin, triphenyltin bromide, triphenyltinchloride, trixylyltin halides, triethylantimony, trimethylantimony, aswell as triphenylantimony, copper acetylacetonate, and ethylcopperacetylacetonate. More preferably, the volatile promoter compound isselected from promoter alkyl, carbonyl, halide, and aryl complexes, andmixed variations thereof, characterized further in that the promotermetal is selected from Groups 1B, 6B, 3A, 4A, 5A, and the lanthanideelements, and more preferably selected from tin, antimony, copper,silver, and indium. The aforementioned examples are used forillustrative purposes only and are not meant to be limiting. One skilledin the art may find other species which are equally suitable.

Any amounts of volatile Group 8B metal compound and/or volatile promotercompound can be fed to the oxidation reactor, provided that the paraffinconversion and the olefin selectivity remain at the desired levels. Thepreferred loadings and atomic ratios of Group 8B metal to promoter areset forth hereinabove. Typically, each volatile compound comprises fromabout 0.1 parts per billion (ppb) to about 5 percent, preferably, fromabout 0.5 parts per million (ppm) to about 1,000 ppm (0.1 percent),based on the total volume of the feedstream. The volatile compounds arefed for a period of time sufficient to deposit the desired amounts ofGroup 8B metal and promoter on the support.

The oxidation process and the catalyst regeneration process of thisinvention are both conducted under autothermal conditions. Thermalenergy is needed to maintain autothermal process conditions. Withoutpreheating the feedstream, the required thermal energy is totallysupplied by the reaction of the feedstream with oxygen, namely,oxidative dehydrogenation to form olefins and water, hydrogen oxidationto form water, and carbon combustion to form carbon monoxide and carbondioxide. Under these conditions the heat generated by the combustion ofa portion of the feed is sufficient to support endothermicdehydrogenation and/or thermal cracking of the paraffin to the olefin.Accordingly, the need for an external heating source to supply theenergy for the process is eliminated. As a requirement for conducting anautothermal process, the catalyst should be capable of combustion beyondthe normal fuel rich limit of flammability. Alternatively, a portion ofthe required thermal energy can be obtained by preheating thefeedstream. The preheat can be conveniently supplied by condensing highpressure saturated steam or by combusting off-gas or other fuel source.Preheat at a temperature greater than about 40° C., but below the onsetof reaction of the feed components can be used without loss in olefinselectivity. As a second alternative, a portion of the required thermalenergy can be obtained by adding hydrogen to the feedstream. Hydrogenreacts exothermically with oxygen to form water. When hydrogen is usedin the feedstream with, optionally, a high preheat, autothermalconditions can be maintained even when the catalyst does not supportcombustion beyond the normal fuel-rich limit of flammability.

Ignition can be effected by preheating the feed to a temperaturesufficient to effect ignition when the feed is contacted with thecatalyst. Alternatively, the feed can be ignited with an ignitionsource, such as a spark or flame. Upon ignition, the reaction-generatedheat causes the temperature to take a step change jump to a new steadystate level which is herein referred to as the autothermal reaction.

While running autothermally, the paraffinic hydrocarbon feed ispreferably preheated to obtain a portion of the thermal energy needed torun the oxidation process. Preheat also volatilizes the Group 8B metalcompound and promoter compound(s), so as to combine them with thereactant feed. Typical preheat temperatures range from about 40° C. toabout 550° C. Preferably, the preheat temperature ranges from about 40°C. to only about 250° C., so as to prevent premature decomposition ofthe volatile Group 8B and promoter compounds in the feed.

As a general rule, the autothermal process operates at close to theadiabatic temperature (that is, essentially without loss of heat), whichis typically greater than about 750° C., and preferably, greater thanabout 925° C. Typically, the autothermal process operates at atemperature less than about 1,150° C., and preferably, less than about1,050° C. Optionally, the temperature at the reactor exit can bemeasured, for example, by using a Pt/Pt-Rh thin wire thermocouple. Witha monolith catalyst, the thermocouple can be sandwiched between themonolith and the downstream radiation shield. Measurement of temperatureclose to the reactor exit may be complicated by the high temperatureinvolved and the fragility of the thermocouple. Thus, as an alternative,one skilled in the art can calculate the adiabatic temperature at thereactor exit from a knowledge of the preheat temperature and the exitstream composition. The "adiabatic temperature" is the temperature ofthe product stream without heat loss, that is, when all of the heatgenerated by the process is used to heat the reactor contents.Typically, the measured temperature is found to be within about 25° C.of the calculated adiabatic temperature.

The operating pressure is typically equal to or greater than about 1atmosphere absolute (atm abs) (100 kPa abs). Typically, the pressure isless than about 20 atm abs (2,000 kPa abs), preferably, less than about10 atm abs (1,000 kPa abs), and more preferably, less than about 7 atmabs (700 kPa abs).

It is desirable to maintain a high space velocity through the reactionzone, otherwise the selectivity to olefinic products may decrease due toundesirable side reactions. The specific gas hourly space velocitiesemployed will depend upon the choice of reactor cross sectionaldimension (for example, diameter), and the form and weight of thecatalyst. Generally, the gas hourly space velocity (GHSV), calculated asthe total flow of the hydrocarbon, oxygen, optional hydrogen, andoptional diluent flows, is greater than about 50,000 ml total feed perml catalyst per hour (h⁻¹) measured at standard temperature and pressure(0° C., 1 atm) (STP). Preferably, the GHSV is greater than about 80,000h⁻¹, and more preferably, greater than 100,000 h⁻¹. Generally, the gashourly space velocity is less than about 6,000,000 h⁻¹, preferably, lessthan about 4,000,000 h⁻¹, more preferably, less than 3,000,000 h⁻¹,measured as the total flow at standard temperature and pressure. Gasflows are typically monitored in units of liters per minute at standardtemperature and pressure (slpm). The conversion of gas flow from "slpm"units to gas hourly space velocity units (h⁻¹) is made as follows:##EQU1## The residence time of the reactants in the reactor is simplycalculated as the inverse of the gas hourly space velocity. At the highspace velocities employed in the process of this invention, theresidence time is on the order of milliseconds. Thus, for example, a gashourly space velocity of 100,000 h⁻¹ measured at STP is equivalent to aresidence time of 36 milliseconds at STP.

The process of this invention may be conducted in any reactor designedfor use under autothermal process conditions, including fixed bed,fluidized bed, and transport reactors. In one preferred design, thecatalyst is prepared on a monolith support which is sandwiched betweentwo radiation shields inside a reactor housing. In another preferredform, a fluidized bed reactor is used with the catalyst in the form ofpellets, spheres, and other particulate shapes. More preferably, thefluidized bed has an aspect ratio less than 1:1 during operation, andeven more preferably, less than 1:1 in static mode, which is theunfluidized or fixed bed configuration. The aspect ratio is the ratio ofthe height (or depth) of the bed to its cross-sectional dimension(diameter or width). Continuous and intermittent flow of the feedstreamare both suitable.

When a paraffinic hydrocarbon is contacted with oxygen under autothermalprocess conditions in the presence of the catalyst describedhereinabove, an olefin, preferably a mono-olefin, is produced. Ethane isconverted primarily to ethylene. Propane and butane are convertedprimarily to ethylene and propylene. Isobutane is converted primarily toisobutylene and propylene. Naphtha and other higher molecular weightparaffins are converted primarily to ethylene and propylene.

The conversion of paraffinic hydrocarbon in the process of thisinvention can vary depending upon the specific feed composition,catalyst, and process conditions employed. For the purposes of thisinvention, "conversion" is defined as the mole percentage of paraffinichydrocarbon in the feed which is converted to products. Generally, atconstant pressure and space velocity, the conversion increases withincreasing temperature. Typically, at constant temperature and pressure,the conversion does not change significantly over a wide range of highspace velocities employed. In this process, the conversion of paraffinichydrocarbon is typically greater than about 45 mole percent, preferably,greater than about 50 mole percent, and more preferably, greater thanabout 60 mole percent.

Likewise, the selectivity to products will vary depending upon thespecific feed composition, catalyst, and process conditions employed.For the purposes of this invention, "selectivity" is defined as thepercentage of carbon atoms in the converted paraffin feed which reactsto form a specific product. For example, the olefin selectivity iscalculated as follows: ##EQU2## Generally, the olefin selectivityincreases with increasing temperature up to a maximum value and declinesas the temperature continues to rise. Usually, the olefin selectivitydoes not change substantially over a wide range of high space velocitiesemployed. In the process of this invention, the olefin selectivity,preferably the combined olefin selectivity to ethylene and propylene, istypically greater than about 50 carbon atom percent, preferably, greaterthan about 60 carbon atom percent, more preferably, greater than about70 carbon atom percent, and even more preferably, greater than about 80carbon atom percent. Other products formed in smaller quantities includemethane, carbon monoxide, carbon dioxide, propane, butenes, butadiene,propadiene, acetylene, methylacetylene, and C₆₊ hydrocarbons. Acetylenecan be hydrogenated downstream to increase the overall selectivity toolefin. Carbon monoxide, carbon dioxide, and methane can be recycled, atleast in part, to the reactor.

Water is also formed in the process of this invention from the reactionof hydrogen or hydrocarbon with oxygen. The presence of hydrogen in thefeed minimizes the formation of carbon oxides by reacting with theoxygen to produce water and energy. Accordingly, it is advantageous torecycle the hydrogen in the product stream, obtained from thedehydrogenation of the paraffin, back to the reactor. Optimally, thehydrogen needed to meet the demands of the process essentially equalsthe hydrogen formed during conversion of the paraffin to olefin. Underthese balanced conditions, the hydrogen forms a closed loop whereinthere is essentially no demand for additional hydrogen to be added tothe feed. Such conditions are more easily met when the feed is preheatedand a higher hydrocarbon to oxygen molar ratio is employed.

Over time the catalyst loses activity due to the loss of catalyticcomponents by vaporization. In the method of this invention a partiallydeactivated catalyst can be easily regenerated on-line during theautothermal oxidation process. A fully-deactivated catalyst can beregenerated on-line under ignition conditions. With this regenerationmethod, there is no need to shut down the process and remove thecatalyst from the reactor. Rather, the regeneration is effected byco-feeding a volatile Group 8B metal compound and/or a volatile promotercompound with the oxidation reactant feed under autothermal or ignitionoperating conditions. Intermittent or continuous feeding of the volatileGroup 8B metal compound and/or the volatile promoter compound are bothsuitable. The volatile compound(s) contact(s) the front face of thecatalyst and decompose(s) at the elevated temperature of autothermal orignition conditions into the corresponding Group 8B metal and/orpromoter(s).

The invention will be further clarified by a consideration of thefollowing examples, which are intended to be purely illustrative of theuse of the invention. Other embodiments of the invention will beapparent to those skilled in the art from a consideration of thisspecification or practice of the invention as disclosed herein. Unlessotherwise noted, all percentages are given on a mole percent basis.Selectivities are given on a carbon atom percent basis.

EXAMPLE 1

Catalyst preparation: An alumina foam monolith support (99.5 weightpercent alumina; 17 mm Outside Diameter (O.D.)×10 mm length; 45 poresper linear inch) was modified with lanthanum by soaking it in an aqueoussolution of lanthanum chloride (1 M). The lanthanum-modified support wascalcined at 900° C., and then further modified with tin by soaking it inan aqueous solution of stannous chloride (0.372 M) to which hydrochloricacid was added to aid solubility. The tin and lanthanum modified supportwas dried at 100° C. and then reduced at 700° C. in flowing hydrogen (5percent hydrogen in nitrogen). The reduced support was then loaded witha solution prepared from stock aqueous solutions of hexachloroplatinicacid (1 ml, 0.193 M) and copper nitrate (0.65 ml, 1.49 M). Platinumloading was 1.0 weight percent. Copper/platinum atomic ratio was 5:1.The loaded monolith was dried at 100° C. and then reduced at 450° C. inflowing hydrogen to yield the catalyst.

Oxidation Process: The catalyst was sandwiched between 4 inert blankalumina monoliths (2 on either side; 17 mm×10 mm) which acted asradiation shields. The five monoliths were wrapped in FiberFrax® brandalumina-silica cloth 1/16 inch (1.6 mm) thick and packed into a quartztube (19 mm Inside Diameter (I.D.)×5 cm length). FiberFrax® is atrademark of the Unifrax Corporation. The quartz tube was then wrappedwith FiberFrax® silica-alumina cloth 1/8 inch (3.2 mm) thick and packedinto a stainless steel reactor 1 inch (2.5 cm) O.D. The feed to thereactor was-preheated with heating tape 10 inches (25 cm) wrapped aroundthe stainless steel reactor upstream of the catalyst. The catalyst zonewas not heated, but was insulated with high temperature insulationmaterial to minimize heat losses. Ethane, hydrogen, and nitrogen werepreheated to 200° C. and fed to the reactor. Oxygen was then introducedto the reactor which resulted in catalyst ignition. Upon ignition thetemperature rose quickly within a few seconds to 1,000° C. and thereactor operated autothermally. Process conditions and results are setforth in Table 1. It was found that the ethane conversion decreased from66.50 percent to 61.95 percent over a period of about 213 h. Over thesame period the selectivity to ethylene decreased from 81.13 to 80.32percent, while the carbon monoxide selectivity increased from 6.07 to8.82 percent.

At 216 h on steam, a first regeneration was conducted as follows.Tetraethyltin (0.4 ml) was added through a septum into an argon lineconnected to the feedstream inlet just before the catalyst. The flow ofargon was maintained at 0.2 slpm. The addition of the volatile tincompound was carried out "on-line" for 20 min during which time thereactor was run autothermally at 1,000° C. at the following conditions:flow, 7 slpm; nitrogen dilution, 20 percent; C₂ H₆ /O₂ molar ratio of 2;H₂ /O₂ molar ratio of 2; 200° C. preheat. A moderately low preheat wasused to prevent decomposition of the vapors of tetraethyltin before theyreached the catalyst. Results after the first regeneration are set forthin Table 1.

It was found that the losses in ethane conversion and ethyleneselectivity were completely recovered by the on-line addition oftetraethyltin. The tetraethyltin (partially or fully vaporized) with theargon entered the reactor just before the catalyst pack and was swept bythe flow of the feed gases through the two upstream radiation shields tothe hot, ignited catalyst where the organotin compound decomposed todeposit tin on the front edge of the catalyst. A sample of the effluenttaken only 20 min after the tin injection showed that the ethaneconversion and ethylene selectivity were recovered to the initial valueat 2.8 h.

After regeneration, the process was run for an additional 291 h up to508.5 h on stream, during which time the ethane conversion and ethyleneselectivity gradually decreased, as shown in Table 1.

After 508.5 h total run time, the reactor design was modified to flow anargon stream (0.1 slpm) continuously over the surface of a reservoir oftetraethyltin at room temperature to obtain continuous regeneration. Theconcentration of tetraethyltin in the argon stream was expected tocorrespond to the vapor pressure of tetraethyltin at room temperature;however, this data was not readily available. Transdecalin has a similarboiling point and flash point as tetraethyltin. Accordingly, the vaporpressure of transdecalin was used to estimate the concentration oftetraethyltin in the argon stream. At 25° C. the vapor pressure oftransdecalin is 1649 ppm. Thus, when the argon stream was mixed with thefeed for the oxidation process (7.5 slpm), the tetraethyltinconcentration was estimated to be approximately 22 ppm. The results ofthe continuous addition are set forth in Table 1 (second regeneration).It can be seen that the ethane conversion was 1 percent higher withcontinuous addition of about 22 ppm tetraethyltin. Higher concentrationsof tetraethyltin should result in further improvements.

After 577.8 h total run time, the continuous addition of tetraethyltinwas turned off and a slug of tetraethyltin (0.5 ml) was added to theargon stream through a septum. As with the first regeneration, theconversion and selectivity improved significantly and the catalyst wasfully regenerated, as shown in Table 1 (third regeneration).

                                      TABLE 1                                     __________________________________________________________________________    Before and After Regeneration.sup.a,b                                                            T                                                            Time C.sub.2 H.sub.6 /  pre-heat % C.sub.2 H.sub.6 % C.sub.2 H.sub.4 %                                             CH.sub.4 % CO % CO.sub.2                 (h) O.sub.2 H.sub.2 /O.sub.2 (° C.) Conv Sel Sel Sel Sel             __________________________________________________________________________     2.8       2.3 2.3 250 66.50                                                                             81.13                                                                             5.96                                                                              6.07                                                                              2.24                                      28.1 2.2 2.2 300 66.65 80.92 5.54 7.88 1.62                                   78 2.2 2.2 300 64.03 80.53 5.44 8.47 1.44                                    126 2.2 2.2 300 63.47 80.28 5.48 8.84 1.37                                    216 2.2 2.2 300 61.95 80.32 5.46 8.82 1.31                                    After first regeneration                                                      (on-line slug):                                                               216.8 2.2 2.2 288 67.00 81.16 6.37 6.44 0.85                                  218 2.2 2.2 300 66.32 81.25 6.09 7.44 0.81                                    Continued run:                                                                291.0 2.2 2.2 300 63.78 80.32 5.81 8.98 0.89                                  508.5 2.2 2.2 300 62.52 80.09 5.53 9.42 0.94                                  Second regeneration                                                           (continuous on-line):                                                         516.2 2.2 2.2 300 63.42 80.37 5.69 9.05 0.81                                  542.8 2.2 2.2 300 63.51 80.36 5.80 9.09 0.81                                  577.8 2.2 2.2 300 63.82 80.37 5.82 8.94 0.78                                  After third regeneration                                                      (on-line slug):                                                               585.7 2.2 2.2 300 69.38 81.03 6.71 6.08 0.70                                __________________________________________________________________________     .sup.a C.sub.2 H.sub.6 /O.sub.2 and H.sub.2 /O.sub.2 given as molar           ratios; Inlet Pressure: 1.35 bar abs; Flow rate: 7.5 slpm; GHSV: 178, 839     h.sup.-1 ; Nitrogen dilution: 10 percent; Calculated adiabatic                temperature: 950-1,050° C.                                             .sup.b % Conv = mole percentage converted ethane; % Sel = percentage          selectivity to product on a carbon atom basis                            

What is claimed is:
 1. An improved process of preparing an olefinwherein a paraffinic hydrocarbon having from 2 to about 25 carbons atomsor mixture thereof is contacted with oxygen in the presence of acatalyst comprising at least one Group 8B metal, and optionally, atleast one promoter metal or metal ion on a support, the contacting beingconducted under autothermal process conditions sufficient to prepare theolefin in a reactor, wherein the improvement comprises simultaneouslyco-feeding into the reactor with the paraffinic hydrocarbon and oxygen,a volatile Group 8B metal compound, and optionally, a volatile promoter,compound continuously or intermittently so as to regenerate the catalyston-line.
 2. The process of claim 1 wherein the paraffinic hydrocarbon isselected from the group consisting of ethane, propane, and mixturesthereof.
 3. The process of claim 1 wherein the paraffinic hydrocarbon isselected from the group consisting of naphtha, natural gas condensates,gas oils, vacuum gas oils, and mixtures thereof.
 4. The process of claim1 wherein the molar ratio of paraffinic hydrocarbon to oxygen rangesfrom about 3 to about 77 times the stoichiometric ratio of hydrocarbonto oxygen for complete combustion to carbon dioxide and water.
 5. Theprocess of claim 1 wherein the molar ratio of paraffinic hydrocarbon tooxygen is about 0.1:1 to about 4.0:1.
 6. The process of claim 1 whereinthe process is operated in the presence of a diluent.
 7. The process ofclaim 6 wherein the diluent is used in an amount of about 0.1 molepercent to about 70 mole percent, based on the total reactant feed. 8.The process of claim 1 wherein, hydrogen is added to the paraffinichydrocarbon.
 9. The process of claim 1 wherein the Group 8B metal is aplatinum group metal.
 10. The process of claim 9 wherein the platinumgroup metal is platinum.
 11. The process of claim 1 wherein the supportis a ceramic monolith support selected from the group consisting ofsilica, alumina, silica-aluminas, aluminosilicates, zirconia, magnesia,magnesium aluminate spinels, magnesium silicates, titania, boria,zirconia toughened alumina, lithium aluminum silicates, silicon carbide,silicon nitride, and oxide-bonded silicon carbide.
 12. The process ofclaim 11 wherein the ceramic monolith support comprises from about 60 toabout 100 weight percent alumina.
 13. The process of claim 1 wherein thepromoter metal or metal ion is selected from the group consisting ofGroups 1A, 2A, 3B, 4B, 5B, 6B, 1B, 3A, 4A, 5A, the lanthanide andactinide elements of the Periodic Table, and mixtures thereof.
 14. Theprocess of claim 1 wherein the atomic ratio of Group 8B metal topromoter metal or metal ion in the fresh catalyst ranges from of about1:10 to about 1:0.5.
 15. The process of claim 1 wherein the process isconducted at a temperature of about 750° C. to about 1,150° C.
 16. Theprocess of claim 1 wherein the process is conducted at a pressure ofabout 1 atm abs to about 20 atm abs.
 17. The process of claim 1 whereinthe process is conducted at a gas hourly space velocity of about 80,000h⁻¹ to about 6,000,000 h⁻¹.
 18. The process of claim 1 wherein theconversion of paraffinic hydrocarbon is about 50 mole percent or higher.19. The process of claim 1 wherein the olefin selectivity is about 60carbon atom percent or higher.
 20. The process of claim 1 wherein thevolatile Group 8B metal compound is a compound or complex containingcarbonyl, alkyl, halo, mono-olefin, diolefin, acetylene, allyl,cyclo(hydrocarbyl)diene, cyclo(hydrocarbyl)dienyl, aryl, alkoxide,oxide, phosphine, or mixtures thereof.
 21. The process of claim 1wherein the volatile promoter compound is a compound or complexcontaining carbonyl, alkyl, halo, mono-olefin, diolefin, acetylene,allyl, cyclo(hydrocarbyl)diene, cyclo(hydrocarbyl)dienyl, aryl,alkoxide, oxide, phosphine, or mixtures thereof.
 22. The process ofclaim 1 wherein the support is pretreated with an element selected fromthe group consisting of Groups 1A, 2A, 3B, 4B, 5B, 6B, 1B, 3A, 4A, 5A,the lanthanide and actinide elements, and mixtures thereof.